This study will characterize by a combination of biochemical, immunological and genetic approaches the inherited variation in activity of galactokinase (GALK) and galactose-l-phosphate uridylyltransferase (GALT) in both man and mouse. The specific aims are: (1) To characterize human variants of GALT both structurally and functionally. (2) To determine the frequency of common and rare variants of GALT in the population. (3) To assess the validity of estimating the frequency of polymorphic alleles with variant enzyme activities by statistically analyzing the distribution of GALT activities in a random population. (4) Monoclonal antibodies of various specificities will be generated to human GALT by mouse hybridoma technique. We expect that these hybridoma reagents will become invaluable in the analysis of human genetic heterogeneity.